Metal treatment – Stock – Aluminum base
Reexamination Certificate
1997-11-28
2001-05-08
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Aluminum base
C075S684000, C075S685000
Reexamination Certificate
active
06228185
ABSTRACT:
This invention relates to metal matrix alloys, and more specifically to metal matrix alloys comprising an aluminium-based matrix having boride ceramic particles dispersed therein.
It has been previously proposed to incorporate particles of ceramic borides such as titanium diboride into aluminium and its alloys to improve their mechanical properties such as stiffness.
Thus, for example, U.S. Pat. No. 3,037,857 (assigned to Union Carbide) teaches making an aluminium-based metal matrix composite by adding pre-formed particles of a boride such as titanium diboride to aluminium or an aluminium alloy. For relatively low boride particle loadings this may be accomplished by adding them to an aluminium melt at about 1200 degrees C. However, the preferred method taught in U.S. Pat. No. 3,037,857 is to dry blend powders of the boride and of the aluminium-based matrix metal cold, compact the blend at high pressure, and then heat to between 1000 and 1150 degrees C. Pre-formed boride particles are expensive. Also, the known techniques for their production inevitably give rise to impurities on their surfaces. This reduces the ability of the particles to be fully wetted by aluminium-based melts, which will adversely affect the mechanical properties of composites made using them.
European Patent Specification No. 0113249 A (Alcan) describes a method of making a metal matrix composite by producing a relatively low loading of ceramic particles such as boride particles by in situ chemical reaction within a melt of a matrix metal such as aluminium or an aluminium alloy. In the process taught in EP 0113249 A, the melt containing the newly-formed ceramic particles is held at elevated temperatures for a sufficient length of time to cause the particles to form an intergrown ceramic network which is said to increase the mechanical strength of the final product. Production of the network normally requires holding at a temperature of at least 1100 degrees C. for a typical period of 30 minutes, and this treatment results in a dramatic reduction in fluidity, so much so that EP 0113249 A recommends carrying out the operation in a crucible having the appropriate shape of the desired final product.
It has now been discovered that it is possible to produce an aluminium-based matrix melt having boride particles dispersed therein which is castable and yet when cast produces a product having surprisingly good mechanical properties.
According to the present invention, there is provided a process for making a castable aluminium-based matrix melt having boride ceramic particles dispersed therein, the process comprising reacting, within an aluminium-based melt, precursors for the particles, so as to produce boride ceramic particles dispersed in the melt, the process being carried out under conditions such that the melt remains fluid.
Preferably, the flow properties of the melt upon completion of the reaction are such that, at temperatures at which the matrix is molten, the melt is not self-supporting. Those flow properties can be controlled by suitable application of the following principles:
(a) As a result of our experience of working with alloys of the kind with which the invention is concerned, we believe that over-heating can cause a loss of fluidity. Therefore, to maintain the melt in a fluid condition, its temperature should be controlled. Preferably, the temperature within the melt should be maintained below 1000 degrees C. throughout the reaction, and indeed subsequently.
(b) The boride particle loading of the product should not be too high. Generally, it should contain less than 15 weight percent, and preferably from 5 to 10 weight percent, of the dispersed boride ceramic particles. We have found that the maximum boride ceramic particle loading that can be incorporated into the melt without it losing its fluidity can vary with the melt's composition. Thus, for example, in virgin aluminium we have obtained pourable melts with up to 15 weight percent of the dispersed ceramic boride particles, whereas in aluminium-silicon alloys we have achieved only up to 10 weight percent. However, the difference may be due more to the temperature regime to which the melt has been subjected than to its composition.
(c) Although less important, we recommend that the product melt should be cast within 30 minutes, and preferably within 10 minutes, of completion of the reaction, as prolonged holding can cause an increase in melt viscosity, i.e. a loss of fluidity.
(d) We believe that stirring can help prevent loss of fluidity of the melt. We therefore recommend that stirring of the melt should be provided, for example by containing the melt within an induction furnace and operating it to provide an inductive stir.
The boride ceramic particles may be any one or more of those of titanium, zirconium, chromium, tantalum, hafnium, niobium, molybdenum and vanadium, titanium diboride being preferred. It is not necessary for the boride ceramic particles to be chemically pure; they may comprise mixed borides (e.g. more than one metal), for example; also, they may comprise one or more boronitrides, for example. Further, other ceramic particles may be present, in addition to the boride ceramic particles.
The reaction within the aluminium-based melt to produce the ceramic boride particles can be any of the many types of reaction procedures known for the in situ production of boride ceramic particles within an aluminium-based melt; several are outlined in the literature relating to the production of titanium-boron-aluminium grain refiners, and also in EP 0113249. It will be appreciated that the reaction will not be of the SHS (self-propagating high temperature synthesis) type, as with such reactions the reaction product is not in the form of a castable melt.
We prefer that the boride particles should be produced by reacting with aluminium in the melt:
(a) a salt which reacts with aluminium to produce boron; and
(b) one or more salts which react with aluminium to produce a boride-forming metal or metals.
Boron produced by reaction of salt (a) with aluminium in the melt will then react with boride-forming metal or metals produced by the reaction of salts(s) (b) with aluminium in the melt, to produce the ceramic boride particles. The reaction can be brought about by feeding, at a controlled rate, a mixture of salts (a) and (b) to the aluminium-based melt, while maintaining stirring of the melt, for example by holding it in a suitably designed and controlled induction furnace. A preferred salt (a) is potassium borofluoride, KBF
4
. We prefer that salt(s) (b) should be one or more double fluorides of potassium and the boride-forming metal(s).
The aluminium-based melt within which the reaction is carried out may be aluminium or an aluminium alloy.
In accordance with a preferred embodiment of the invention, the boride ceramic particles comprise particles comprising titanium diboride, and we prefer that the weight ratio of titanium to boron in the product should be from 2.5:1 to 2:1, preferably from 2.3:1 to 2.1:1.
The preferred method of performing the preferred embodiment described in the previous paragraph is to produce the boride particles by reacting within the melt potassium borofluoride, KBF
4
, and a potassium fluorotitanate, preferably potassium hexafluorotitanate, K
2
TiF
6
. The two salts are preferably fed to the aluminium-based melt at a controlled rate, while maintaining stirring of the melt, preferably in the manner described above.
By in situ production of the boride ceramic particles in accordance with the process of the invention, it is possible to produce a castable melt product in which the majority of the boride ceramic particles are less than 1 micron in size, as determined under an optical microscope.
Once the castable melt comprising boride ceramic particles dispersed in metal matrix melt has been produced, it can be cast, by conventional means.
If necessary, the composition of the matrix metal may be adjusted before casting, to give the required final composition. It may be desirable to make such an adjustment of the m
Davies Peter
Kellie James Leslie Frederick
Parton Douglas Philip
Wood John Vivian
Fish & Neave
Huang Eric H.
Ingerman Jeffrey H.
London & Scandinavian Metallurgical Co., Ltd.
Wyszomierski George
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